The invention relates in general to the field of microfluidic surface processing devices and in particular to distance-to-surface control systems for such devices.
Microfluidics generally refers to microfabricated devices, which are used for moving, sampling, mixing, analyzing and dosing liquids. Prominent features thereof originate from the peculiar behavior that liquids exhibit at the micrometer length scale. Flow of liquids in microfluidics is typically laminar. Volumes well below one nanoliter can be reached by fabricating structures with lateral dimensions in the micrometer range. Reactions that are limited at large scales (by diffusion of reactants) can be accelerated. Finally, parallel streams of liquids can possibly be accurately and reproducibility controlled, allowing for chemical reactions and gradients to be made at liquid/liquid and liquid/solid interfaces. Microfluidics are accordingly used for various applications in life sciences. Many microfluidic devices have user chip interfaces and closed flowpaths. Closed flowpaths facilitate the integration of functional elements (e.g. heaters, mixers, pumps, UV detector, valves, etc.) into one device while minimizing problems related to leaks and evaporation.
Devising systems and methods for controlling and regulating the distance of microfluidic surface processing devices to the processed surface is particularly challenging: distance control in the tens and hundred of micrometer scale is required. This is even more difficult in the context of life-sciences since the surfaces are most often immersed in liquids. The surfaces may also exhibit strong variations in the topography, composition, and may not be flat even in the macroscopic world.
The known distance control systems/methods require external control systems, as well as, in some cases, pressure regulation. For example, it is known to use a beam deflection system, using a laser and photodetector to measure the beam position. A feedback loop keeps a preset amplitude constant by adjusting the piezo-length in z direction. It is, however, difficult to focus the laser in a liquid environment, and with a liquid air interface. In other, simpler approaches, an operator visually observes the sample and makes changes accordingly. In such cases the operator is the “control” system; this approach obviously suffers from a slow time response; the precision achieved is likely questionable. Still other approaches rely on a priori calibrations. Such calibrations are however specific to particular surface topographies and are time consuming. In addition, some liquid-surface interactions will not be easy, or even not possible, to visualize.
Still other approaches consist of controlling the distance of devices under liquid using current or voltage measurements with a closed-loop system. However, such approaches require a Faraday cage and are not suitable for longer distances in the tens of micrometers, as currents needed for longer distances would cause undesired heating of the system.
There is accordingly a need for simple, yet precise distance control systems and methods for microfluidic surface processing devices.